1. Field of the Invention
The present invention relates to determining position of a tool for operations at a location of the tool. In particular, the present invention relates to calibrating sensors for determining position of the sensors. Even more particularly, the present invention relates to calibrating sensors on a downhole tool to more accurately determine position of the downhole tool in an underground formation, such that setting location of activity or operations, such as drilling by the downhole tool, is more accurate.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Sensors to determine position are used in a tremendous number of important processes, such as deployment of vessels in space, movement of a video game controller, and injection of cells in a tumor. On large scales and small scales, determining position for operations at the determined position can be very important. Accurate sensors are crucial for the performance of those activities. Sensors are also known to direct oil and gas operations in a rock formation. The direction of the tool and the location of the wellbore are detected, so that the various downhole activities can be accurately placed in the formation. These downhole activities include drilling, injecting, and isolating zones in the formation. The accuracy of the sensor and placement of a wellbore can seriously affect the outcome of a drilling operation.
Sensors are calibrated to increase the amount of accuracy and precision so that the determination of position is also as accurate and precise as possible. Sensors in extreme environmental conditions are subject to error, due to those conditions, such as high temperatures. The environment of a sensor can include the depth, pressure and heat in a wellbore. Alternative environments for accelerometers also include high temperatures from electronic components in a circuit board of a video game controller, re-entry heat in orbit, and elevated temperatures from radiation treatment in cellular tissue
In the prior art, redundancy is used for increasing accuracy. U.S. Pat. No. 6,206,108, issued to MacDonald, et al. on Mar. 27, 2001, teaches a method for adjusting a drilling operation based on a system with multiple sensors to measure multiple parameters. The sensors correct each other, and each sensor measurement further refines an instrument reading downhole. In U.S. Patent Application No. 2010/0078216, published for Radford, et al. on Apr. 1, 2010, a system and method for downhole vibration monitoring for reaming tools includes a plurality of accelerometers, a plurality of magnetometers, and at least one temperature sensor. The plurality of accelerometers corrects or verifies other sensors to guide drilling. U.S. Pat. No. 6,648,082, issued to Schultz, et al. on Nov. 18, 2003, teaches a method for differential sensor measurement and a system to detect drill bit failure. The system incorporates a main sensor and individual sensors for other sensor values, which are compared to each other to create a self-correcting system.
The prior art further establishes mathematical models to increase accuracy. U.S. Pat. No. 8,818,779, issued to Sadlier, et al. on Aug. 26, 2014, teaches a system and method for real-time wellbore stability while drilling a borehole. The drilling operation is adjusted in real time according to sensor readings compared against a geomechanical model. U.S. Patent Application No. 2014/0231141, published for Hay, et al. on Aug. 21, 2014, discloses a system and method for automatic weight on bit sensor correction with a sensor arranged in a bottomhole assembly. The method comprises first taking a survey recording an initial depth within a borehole, calculating a prediction borehole curvature at a second depth, calculating a weight correction value based on the predicted hole curvature, and finally adjusting the borehole position with the weight correction value.
Adjusting for accuracy in the prior art focuses on external factors and conditions affecting readings, not the sensor itself. The mathematic models apply to a specific context for drilling operations, not the general accuracy and precision of the sensor. In a different context for a different activity (deep space, microsurgery), there is still a need to calibrate according to the components of the sensor itself.
The factory calibration of a sensor is addressed in the prior art. Upon assembly, the components of the sensor are calibrated before applied in a specific context with other distorting external conditions. U.S. Pat. No. 5,880,680, issued to Wisehart, et al. on Mar. 9, 1999, teaches calibration of a sensor according to a temperature model. One accelerometer is tested at the time of manufacture to determine a temperature model of how accuracy of the accelerometer is affected at different temperatures. In the drilling operation, a temperature sensor and the accelerometer are run in the wellbore, and the readings of the accelerometer and the readings from the temperature sensor are processed according to the temperature model. U.S. Pat. No. 7,234,540, issued to Estes, et al. on Jun. 26, 2007, teaches a system and method of a two-axis gyroscope and other sensors which, when incorporated into a bottomhole assembly, determines the direction of the wellbore and drilling tool in real-time. A number of corrective operations are applied to the sensors while downhole, including a scale factor correction for the temperature at the final position.
The prior art general calibration requires numerous measurements taken over many sensor orientations and temperatures, and the prior art methods only account for temperature affecting components of the sensors. However, temperature is not the only factor, especially for certain types of sensors.
More modern accelerometers are small micro electro-mechanical systems (MEMS or micro-mechanical systems, MMS). One of the most simple and less expensive MEMS devices is an open loop MEMS device, which basically consists of a hinged micro machined silicon wafer. The silicon wafer is the sensing element that moves in the presence of a gravitational field or acceleration force. The open loop MMS sensor measures the departure from a neutral starting position of the wafer. Another MEMS device is a closed loop MEMS device basically consisting of a cantilever beam, such as a cantilever beam, and a proof mass on the beam. The cantilever beam can be maintained in a neutral zero force position by applying a current flow through a small magnetic element, which creates the exact force to neutralize the gravitational force acting on the cantilever beam. An additional amount of current is applied proportionally to the gravitational field vector being measured in a particular orientation as required to keep the cantilever beam in the neutral position. A magnetic force induced by the electric current returns the cantilever beam to the start or neutral position. Thus, by measuring the amount of current to return the cantilever beam to the neutral force position, the amount of acceleration force or gravitational force can be measured. The present invention is applicable to both open loop and closed loop sensors. There is particular utility for open loop sensors with the present invention.
The errors from MMS sensors can originate from bias and hysteresis type distortion. Bias error happens because the cantilever beam is deformed by high temperatures and cannot return to the same neutral/null position with the same current induced magnetic force. Hysteresis type distortion happens because the amount of deformation of the cantilever beam by high temperatures relates to the time spent at the high temperatures. The physical component, such as the silicon wafer and cantilever beam, further deforms from being exposed to the repeated higher and lower temperatures for different amounts of time. These components of the MEMS accelerometer are affected by additional errors, besides the temperature itself. The long term effects of high temperature are not addressed in the current calibration methods, which have particularly high impact on MEMS accelerometers.
Bias errors and hysteresis errors are known for prior art accelerometers. The '540 patent also includes a bias correction, which is obtained from a prior survey; and misalignment and gravity dependent corrections to the gyroscope axes. Use of a sensor will have bias errors, as the components drift when the sensor is in use. However, the MMS sensors, in particular open loop MMS sensors, are more susceptible to bias errors and hysteresis errors, especially with exposure to high temperatures over extended periods of time. MMS sensors can be calibrated according to the prior art, including attempts at bias correction; however, the prior art for regular accelerometers does not address MMS sensors, which are more prone to these errors than regular accelerometers. The sensitivity and durability for the components of MMS sensors are not the same as traditional accelerometers. The old solutions for error of the prior art can apply to MMS sensors, but those old solutions are not sufficient for reliability. Additional solutions are required for the MMS sensors to be used repeatedly in high temperature conditions.
The accuracy problem of MMS sensors is known, and the shortcomings of a factory temperature model calibration is also known. U.S. Pat. No. 7,168,507, issued to Downton on Jan. 30, 2007, teaches a system and method for recalibrating downhole sensors by comparing output values of two sets of sensors. The first set of sensors is inexpensive and comprised of less accurate MMS sensors, so they are placed close to the drill bit with a high risk of damage. The second set of sensors is expensive and placed in a more stable remote location. The second set of sensors are accelerometers measuring the same parameters, when the second set arrives at the same location where the first set took measurements. The first set of MMS sensors are calibrated by the readings from the second set. Instead of solving the reliability problem of the MMS sensors, the prior art solution is to retain a second set of the more expensive accelerometers to double check the MMS sensors. The '507 patent acknowledges the known error rate of inexpensive MMS sensors, but the solution of adding a second expensive set of more accurate sensors remains expensive and redundant.
It is an object of the present invention to provide a method for determining position with improved calibration.
It is another object of the present invention to provide a method for determining position of a tool or a terminal device at a location for initiating activity.
It is still another object of the present invention to provide a method for initiating activity at a particular location determined by a calibrated sensor.
It is an object of the present invention to calibrate a sensor.
It is another object of the present invention to calibrate an MMS sensor.
It is still another object of the present invention to calibrate an MMS sensor for a bias correction.
It is yet another object of the present invention to calibrate an MMS sensor for a hysteresis correction.
It is another object of the present invention to provide a method for determining position with improved calibration of an MMS sensor.
It is still another object of the present invention to provide a method for initiating activity at a particular location determined by a calibrated MMS sensor.
It is an object of the present invention to provide a method for generating a plastic bias value for calibrating an MMS sensor.
It is another object of the present invention to provide a method for determining a plastic bias value based on temperature, time duration at a temperature, and gravity.
It is an object of the present invention to calibrate a sensor after a factory calibration of the sensor.
It is an object of the present invention to calibrate a sensor with a plastic bias value concurrently with a factory calibration of the sensor.
It is an object of the present invention to continuously calibrate a sensor with an adjusted plastic bias value.
It is another object of the present invention to provide a method for determining position with a continuous calibration of an MMS sensor.
It is still another object of the present invention to provide a method for initiating and maintaining activity at particular locations determined by a continuously calibrated MMS sensor.
These and other objectives and advantages of the present invention will become apparent from a reading of the attached specification.